Editor’s note:This new feature from Bayer MaterialScience, an industry leader in polyurethane chemistry, will provide formulating help to readers. Their team of experts can help you improve green strength and heat resistance, or adjust the reactivity of an adhesive, as well as provide solutions to improve the tensile strength, elongation, and UV stability of a sealant. Questions regarding contact bonding and laminating with polychloroprene can be addressed as well. Please see below to find out how to submit a question.
In a previous column, the preparation of MDI-based prepolymers was discussed. What are the reasons for choosing monomeric 4,4'-MDI vs. MDI monomer enriched in the 2,4'-MDI isomer or polymeric MDI as the starting isocyanate for the prepolymer?Monomeric MDI-based prepolymers will generally produce a lower viscosity prepolymer than a prepolymer based on a polymeric MDI if they are reacted to the same final isocyanate content. This is because monomeric MDI has a functionality of 2, while polymeric MDI has a functionality of ~2.3 to 3.0, depending on the grade used. The higher functionality starting isocyanate will generally produce a higher viscosity prepolymer. A prepolymer based on a polymeric MDI isocyanate will generally cure faster than a prepolymer based on monomeric MDI. The higher functionality leads to a polymerization process that rapidly forms crosslinked networks and produces faster gelation.
Prepolymers based on 2,4’-MDI have some distinct performance differences compared to those based solely on 4,4’-MDI. The prepolymers containing 2,4’-MDI usually have lower viscosity and cure at a slower rate than prepolymers based on 4,4’-MDI. In addition, the prepolymers based on 2,4’-MDI are less likely to crystallize at low temperatures, which is an advantage when shipping products during the winter months. These differences are due to the steric hindrance of the isocyanate group at the 2-position. Prepolymers containing some polymeric MDI also have improved low-temperature stability, which is related to the irregular nature of the prepolymer backbone.
Our group is trying to develop adhesives for new applications using waterborne polyurethanes. What formulation recommendations can you suggest?Polyurethane adhesive dispersions can be separated into two broad categories: soft and amorphous, or harder and more crystalline. The softer, more amorphous polymers have traditionally been used in wet-bonding applications, such as textile bonding. The amorphous polymer provides a soft feel to the resulting bond.
A harder, more crystalline polymer would produce a more inflexible bond line. Crystalline polymers have been used to bond films to more rigid substrates, such as plastic or wood, as well as in shoe applications that require a strong, flexible bond. These more crystalline films have demonstrated rapid bond-strength development and good plasticizer resistance.
Of course, dispersions of amorphous and crystalline polyurethanes can be blended to produce an intermediate level of performance. These dispersions can be used for bonding flexible films in food packaging, for example.
Remember, using dispersions with a little or no co-solvent content will optimize the “green” aspect of your adhesive.
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